1. Field of the Invention
The present invention relates to a lifting apparatus capable of moving a platform vertically above a chassis so as to raise and lower an operator, an object or a material located on the platform and, more particularly, to a lifting apparatus having a simple structure composed of one telescopic boom body and, yet, which can function in a manner equivalent to that of a conventional lifting apparatus having plural telescopic boom bodies, and which is capable of lengthwise extension and contraction at a rate correlated to the angle of inclination of the telescopic boom body with respect to the vertical.
2. Description of the Prior Art
Lifting apparatuses are widely used for assembling, painting and repairing highway bridges, building constructions or the like, which are in elevated locations. In such apparatuses, an operator, an object or a material is placed on a platform which is then raised or lowered.
A conventional lifting apparatus comprises a plurality of groups of arms, wherein each group of arms comprises a pair of arms which are pivotally connected at the central portion thereof. The plurality of groups of arms are assembled as one unit for forming a pantograph by combining the plurality of groups of arms vertically (a so-called scissors-type lifting apparatus). In the conventional arrangement of such an apparatus, it is necessary to lengthen each arm or to increase the number of groups of arms to be connected with one another in order to increase the height to which the platform can be raised. Accordingly, if a lifting apparatus capable of raising a platform to a higher position is designed, a plurality of groups of pantographs are required. This involves the problem that when the lifting apparatus is in its collapsed state wherein the linkage is folded, the platform is higher than is desired and the operation of loading the operator or the material is troublesome.
There was proposed another lifting apparatus capable of stretching one arm in the longitudinal direction thereof by inserting a plurality of booms stretchably into an arm (as disclosed in, e.g., Japanese Patent Application No. 56-134487 and No. 56-191065). In that lifting apparatus, middle booms comprising one group composed of a pair of middle booms are rotatably assembled at the central portion thereof in an X-shape, and two groups of middle booms are arranged in parallel with each other wherein an upper boom and a lower boom are respectively inserted into each middle boom so as to connect the chassis to the platform. This lifting apparatus has the problem that the numbers of booms are increased and the numbers of components are also increased, which involves laborious work for manufacture and assembly thereof, with consequent high cost.
In that apparatus, the sliding portions of each boom are increased which requires slidable parts composed of synthetic resins, such as polyamide, for keeping in good condition the zone in which the sliding portions slide. These sliding parts should be regularly replaced with new parts. This involves an increase of the number of sliding parts and laborious work for inspection and maintenance, and high cost thereof.
To solve those problems, there was proposed another lifting apparatus comprising one elongatable boom and forming a Z-shape viewed from the side (Japanese Patent No. 59-95797). In this mechanism, it is necessary to control the direction in which the one elongatable boom extends and to control the inclination angle for inclining the one elongatable boom upwardly and downwardly, wherein both controls should be made to operate in synchronism with each other. Both controls necessitate a telescopic measuring unit for measuring the elongation amount of a telescopic boom body and an angle measuring unit for measuring the inclination angle of the telescopic boom body relative to the horizontal, wherein both units issue detecting signals which are calculated so as to control a first hydraulic cylinder for adjusting the inclination angle and a second hydraulic cylinder for controlling telescoping of the boom. It is complex to arrange these two measuring units in the lifting apparatus in view of the complicated assembly thereof. Furthermore, a calculating computer, such as microcomputer and the like, is required for calculating the detected signals issued by the two measuring units. The measuring units and the computer, respectively, are high cost items, which result in an increase of the manufacturing cost of the lifting apparatus as a whole. The cost of the measuring units and the computer do not greatly influence the total cost of a large size lifting apparatus, but they significantly influence the total cost of a small size lifting apparatus because the cost price ratio of the computer is high relative to the total cost of the small size lifting apparatus.
The proposed Z-shaped lifting apparatus has the advantages that it requires fewer components compared with the conventional scissors-type lifting apparatus and the X-shaped lifting apparatus. However, this Z-shaped lifting apparatus has a drawback that the controlling mechanism is complex and involves high cost because the telescopic boom body should be controlled in respect of inclination angle and the lengthwise extension and contraction.
Accordingly, it is desired to provide a simplified control mechanism capable of lifting the platform vertically relative to the chassis without rotating two measuring units for measuring the elongation of the telescopic boom body and the inclination angle of the telescopic boom body and without providing a computer for calculating the detecting signals issued by these measuring units. Particularly, the control mechanism can mechanically control the platform relative to the chassis without resorting to electronic instrument apparatuses, such as high-priced computers.